Method of recovering impregnating agent compositions

ABSTRACT

The present invention relates to processes for recovering impregnating compositions, the impregnated metal part being brought into contact with a salt-containing cleaning composition which comprises at least 0.7% by weight of at least one salt. 
     In addition, the present invention describes processes for impregnating metal parts, which have at least one step for recovering impregnating compositions.

FIELD OF THE INVENTION

The present invention relates to processes for recovering polymerizableimpregnating compositions. Furthermore, the present invention describesprocesses for impregnating metal parts.

Various metal parts or metal components, such as, for example, engineblocks, cylinder heads, gear housings, etc., are produced by casting ofvarious metals or metal alloys. In general, the metal parts thusproduced have pores which may lead to leaks. Great value is increasinglybeing placed on a low weight of the metal parts. In addition, theproduction of thin-walled metal parts leads to a saving of material.Accordingly, the necessity of sealing the pores formed by the casting isgrowing.

PRIOR ART

Impregnating compositions which comprise mixtures of esters of(meth)acrylic acid have long been used for this purpose. However, only asmall part of the impregnating compositions used is actually used forsealing the pores. A major part is removed from the metal surface bywashwater before the impregnating composition present in the pores ishardened. The impregnating composition present in the washwater mustthen be disposed of in a complicated manner.

In order to solve this problem, the prior art proposes using specificimpregnating compositions which have a density of more than 1.0. This isdescribed, for example, in the publications U.S. Pat. No. 6,761,775 andU.S. Pat. No. 6,712,910. A disadvantage of this procedure is, however,that only very specific impregnating compositions can be used. As aresult, the quality of the metal parts may be adversely affected sincethe properties of the pore seal, for example the resistance thereof tochemicals, is dependent to a high degree on the impregnatingcomposition.

In addition, the publication U.S. Pat No. 5,518,632 discloses a processfor recovering impregnating composition, in which the washwater istreated by addition of an additive which increases the density of thewater. As a result, water-soluble constituents of the impregnatingcompositions are separated from the washwater. However, a disadvantageof this process is that large amounts of washwater are required, whichhave to be purified in a complicated manner. In addition, large amountsof additive, for example sodium chloride, are required. Furthermore, therecovery of the impregnating agent by the process described in U.S. PatNo. 5,518, 632 cannot be operated continuously.

Furthermore, the publications DE-A-44 10 193 and DE-A-101 13 278disclose processes in which the impregnating agent is separated off fromthe washwater via reverse osmosis methods. A disadvantage here, however,is that the process is very energy-consumptive, the plants giving riseto a very high maintenance effort.

In addition, the publications JP-A-2004204201, JP-A-2005076014 andJP-A-2005132971 disclose impregnating compositions which can easily beseparated off from the washwater. The wash compositions may contain avery specific surfactant. However, a disadvantage is that this teachingleads to satisfactory separation only with very specific compositions,so that the statements made above also apply to these publications.

OBJECT AND ACHIEVEMENT

In view of the prior art, it was now an object of the present inventionto provide processes for recovering impregnating compositions which canbe used for recovering a multiplicity of different impregnatingcompositions.

A further object of the invention was to provide a process in which therecovery of the impregnating agent can be carried out continuously.

In addition, it was an object of the present invention to provideprocesses for recovering impregnating compositions which can be carriedout easily and economically. The process should be capable of beingcarried out thereby with a low water consumption, it being intended toavoid the use of particularly large amounts of additive.

Furthermore, the provision of processes for impregnating metal parts wasan object of the present invention. In particular, this process shouldbe capable of being carried out with a multiplicity of differentimpregnating compositions, in order thus to ensure an optimum quality ofthe impregnated metal parts. This process should thereby consume assmall amounts of impregnating agent as possible.

These objects and further objects which are not explicitly mentioned butcan be directly derived or concluded from the contexts discussed in thisintroduction are achieved by a process having all features of PatentClaim 1. Expedient modifications of the process according to theinvention are protected in the dependent claims relating back to Claim1. With regard to the process for impregnating metal parts, Claim 17provides an achievement of the underlying object.

The present invention accordingly relates to a process for recoveringimpregnating compositions, which is characterized in that theimpregnated metal part is brought into contact with a salt-containingcleaning composition which comprises at least 0.7% by weight of at leastone salt.

As a result, it was possible in an unforeseeable manner to provide aprocess for recovering impregnating compositions, which process can beused for recovering a multiplicity of different impregnatingcompositions.

Furthermore, the process according to the invention makes it possible tocarry out the recovery of the impregnating agent continuously.

In addition, the process for recovering impregnating compositions can becarried out easily and economically. The water consumption can be keptlow thereby, it also being possible to avoid the use of particularlylarge amounts of additive.

Furthermore, the process according to the invention can be carried outas a part-step in a process for impregnating metal parts. Surprisingly,it is thus possible to provide a process for impregnating metal partswhich can be carried out with a multiplicity of different impregnatingcompositions. By the easy and economical recovery of excess impregnatingcomposition, this process provides a very environmentally friendlyvariant for impregnating metal parts, these advantages also beingachieved with the use of monomer compositions which are hazardous tohealth.

The process according to the invention serves for recoveringimpregnating compositions. In the case of the customary processes forimpregnating metal parts, only a small part of the impregnatingcomposition is used for filling the pores. A greater part, in generalmore than 90%, is removed from the metal surface by cleaningcompositions, for example washwater. The present process now serves forrecovering the impregnating composition used in excess. According to theinvention, for this purpose, the impregnated metal part is brought intocontact with a salt-containing cleaning composition. Here, theexpression “impregnated metal part” means that the pores of the metalpart are filled with impregnating composition, the metal part generallybeing brought into contact prior to hardening of the impregnatingcomposition with the salt-containing cleaning composition in order toremove excess impregnating composition from the metal surface.

According to the invention, the cleaning composition comprises at leastone salt. The expression “salt” is furthermore known among those skilledin the art and designates an ionic compound which may be of an organicand/or inorganic nature, inorganic salts being preferred. In contrast toinorganic salts, organic salts have hydrocarbon groups. Preferably, thesalt should be environmentally compatible and economical. Expedientsalts have only a slight emulsifying effect, particularly preferably noemulsifying effect. In addition, the salt should as far as possible notlead to corrosion of the metal part. The preferred salts include inparticular halides, for example chlorides, bromides and fluorides,sulphates, sulphites and salts of phosphorus, for example phosphates,phosphonates or phosphinates. Preferred salts may comprise in particularalkali metals, for example sodium, lithium or potassium, and alkalineearth metals, for example magnesium or calcium, as a cation. Forexample, sodium chloride (NaCl), potassium chloride (KCl), sodiumsulphate (Na₂SO₄) and/or potassium sulphate (K₂SO₄) can be used.Particularly preferably, the cleaning compositions of the presentinvention comprise in particular phosphates of alkali metals, such as,for example, sodium phosphate or potassium phosphate, it being possiblefor these salts, depending on pH, also to be present as sodiumdihydrogen phosphate NaH₂PO₄, disodium hydrogen phosphate Na₂HPO₄,potassium dihydrogen phosphate KH₂PO₄ and/or dipotassium hydrogenphosphate K₂HPO₄. These salts can also be used as a mixture.

The salt content of the cleaning composition is preferably in a rangefrom 4 to 20% by weight, particularly preferably in a range from 8 to15% by weight, the salt being present in this amount in solution in thecleaning composition. The cleaning composition preferably compriseswater as the main constituent.

Of particular interest are especially cleaning compositions whichpreferably have a density in the range from 1.005 to 1.20 g/l,particularly preferably in the range from 1.04 to 1.16 g/l.

Impregnating compositions for impregnating metal parts have long beenknown, it being possible for each of these impregnating compositions inprinciple to be recovered by the present process. Especiallyimpregnating compositions which comprise (meth)acrylates areparticularly expedient.

The expression (meth)acrylates includes methacrylates and acrylates andmixtures of the two. The (meth)acrylates present in the impregnatingcompositions have long been known, these monomers being described, interalia, in the publications U.S. Pat. No. 6,761,775 and U.S. Pat No.6,712,910.

Preferred impregnating compositions may comprise, inter alia,hydroxyalkyl (meth)acrylates. These include, inter alia, 2-hydroxyethyl(meth)acrylate (HEMA), 2-hydroxypropyl (meth)acrylate (HPMA),3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate,3,4-dihydroxybutyl (meth)acrylate.

Expedient embodiments comprise in particular 2-hydroxyethyl methacrylateand/or 2-hydroxypropyl methacrylate.

An impregnating composition can preferably comprise 5% by weight to 95%by weight, particularly preferably 25 to 60% by weight, based on theweight of the impregnating composition, of hydroxyalkyl (meth)acrylates.

Of particular interest are furthermore impregnating compositions whichcontain crosslinking (meth)acrylates. These include in particular(meth)acrylates having two double bonds, such as, for example,(meth)acrylates which are derived from unsaturated alcohols, such as,for example, 2-propynyl (meth)acrylate, allyl (meth)acrylate, vinyl(meth)acrylate, and (meth)acrylates which are derived from diols oralcohols having a higher hydricity, such as, for example, glycoldi(meth)acrylates, such as ethylene glycol di(meth)acrylate, diethyleneglycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetra- andpolyethylene glycol di(meth)acrylate, in particular polydiol-400di(meth)acrylate, 1,3-butanediol (meth)acrylate, 1,4-butanediol(meth)acrylate, neopentyl glycol di(meth)acrylates, 1,6-hexanedioldi(meth)acrylate, glyceryl di(meth)acrylate and diurethanedimethacrylate; (meth)acrylates having three or more double bonds, suchas, for example, glyceryl tri(meth)acrylate, trimethylolpropanetri(meth)acrylate, pentaerythrityl tetra(meth)acrylate anddipentaerythrityl penta(meth)acrylate.

Preferred modifications comprise, for example, 5% by weight to 75% byweight, particularly preferably 20% by weight to 50% by weight, ofcrosslinking (meth)-acrylates, based on the weight of the impregnatingcomposition.

Furthermore, preferred impregnating compositions may comprise 5% byweight to 60% by weight, particularly preferably 20% by weight to 40% byweight, of comonomers, based on the weight of the impregnatingcomposition. Comonomers are monomers which can be copolymerized with theabovementioned (meth)acrylates. These include, for example,(meth)acrylates which are derived from saturated alcohols. Thesemonomers comprise in particular (meth)acrylates having 1 to 6 carbonatoms in the alcohol radical, such as, for example, methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl(meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate andpentyl (meth)acrylate, hexyl (meth)acrylate; (meth)acrylates having 7 to15 carbon atoms in the alcohol radical, such as, for example,2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, 2-tert-butylheptyl(meth)acrylate, octyl (meth)acrylate, 3-isopropylheptyl (meth)acrylate,nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate,5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, 2-methyldodecyl(meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl(meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate;(meth)acrylates having 16 to 30 carbon atoms in the alcohol radical,such as, for example, hexadecyl (meth)acrylate, 2-methyl-hexadecyl(meth)acrylate, heptadecyl (meth)acrylate, 5-isopropylheptadecyl(meth)acrylate, 4-tert-butyloctadecyl (meth)acrylate, 5-ethyloctadecyl(meth)-acrylate, 3-isopropyloctadecyl (meth)acrylate, octadecyl(meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate,cetyleicosyl (meth)acrylate, stearyleicosyl (meth)acrylate, docosyl(meth)acrylate and/or eicosyltetratriacontyl (meth)acrylate.

Furthermore, the impregnating compositions may comprise (meth)acrylatesof ether alcohols, such as tetrahydrofurfuryl (meth)acrylate,vinyloxyethoxyethyl (meth)acrylate; amides and nitriles of (meth)acrylicacid, such as N-(3-dimethyl-aminopropyl) meth)acrylamide,N-(diethylphosphono)(meth)acrylamide,(1-methacryloylamido-2-methyl-2-propanol; cycloalkyl (meth)acrylates,such as 3-vinylcyclohexyl (meth)acrylate, bornyl (meth)acrylate; aryl(meth)acrylates, such as benzyl methacrylate or phenyl methacrylate, itbeing possible for the aryl radicals in each case to be unsubstituted orup to tetrasubstituted; carbonyl-containing methacrylates, such as2-carboxyethyl methacrylate, carboxymethyl methacrylate,oxazolidinylethyl methacrylate, N-(methacryloyloxy)formamide; acetonylmethacrylate, N-methacryloylmorpholine, N-methacryloyl-2-pyrrolidinone,N-(2-methacryloyloxyethyl)-2-pyrrolidinone, N-(3-methacryloyloxypropyl)-2-pyrrolidinone, N-(2-methacryloyloxypentadecyl)-2-pyrrolidinone,N-(3-methacryloyloxyheptadecyl)-2-pyrrolidinone; glycidyl methacrylate;phosphorus, boron and/or silicon-containing methacrylates, such as2-(dimethylphosphato)propyl methacrylate, 2-(ethylenephosphito)propylmethacrylate, dimethylphosphinomethyl methacrylate,dimethylphosphonoethyl methacrylate, diethyl methacryloyl phosphonate,dipropyl methacryloyl phosphate, 2-(dibutylphosphono)ethyl methacrylate,2,3-butylenemethacryloylethyl borate,methyldiethoxymethacryloylethoxysilane, diethylphosphatoethylmethacrylate; vinyl halides, such as, for example, vinyl chloride, vinylfluoride, vinylidene chloride and vinylidene fluoride; heterocyclic(meth)acrylates, such as 2-(1-imidazolyl)ethyl (meth)acrylate,2-(4-morpholinyl)ethyl (meth)acrylate and1-(2-methacryloyloxyethyl)-2-pyrrolidone; vinyl esters, such as vinylacetate; styrene, substituted styrenes having an alkyl substituent inthe side chain, such as, for example, α-methyl-styrene andα-ethylstyrene, substituted styrenes having an alkyl substituent on thering, such as vinyl toluene and p-methylstyrene, halogenated styrenes,such as, for example, monochlorostyrenes, dichlorostyrenes,tribromostyrenes and tetrabromostyrenes; heterocyclic vinyl compounds,such as 2-vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine,3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, vinylpyrimidine,vinylpiperidine, 9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole,1-vinylimidazole, 2-methyl-1-vinylimidazole, N-vinylpyrrolidone,2-vinylpyrrolidone, N-vinylpyrrolidine, 3-vinylpyrrolidine,N-vinylcaprolactam, N-vinylbutyrolactam, vinyloxolane, vinylfuran,vinylthiophene, vinylthiolane, vinylthiazoles and hydrogenatedvinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles; vinylethers and isoprenyl ethers.

Preferred comonomers are, inter alia, (meth)acrylates which are derivedfrom saturated alcohols having 7 to 15 carbon atoms or 16 to 30 carbonatoms.

The abovementioned hydroxyalkyl (meth)acrylates, crosslinking(meth)acrylates and comonomers can be used individually or as a mixture.

Preferred impregnating compositions comprise, for example, 5% by weightto 95% by weight, preferably 25 to 60% by weight, of hydroxyalkyl(meth)acrylates, in particular 2-hydroxyethyl methacrylate and/or2-hydroxypropyl methacrylate; 5% by weight to 75% by weight, preferably20% by weight to 50% by weight, of crosslinking (meth)acrylates, inparticular ethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylatetriethylene glycol dimethacrylate, tetraethylene glycol dimethacrylateand/or trimethylolpropane trimethacrylate; and 5% by weight to 50% byweight, preferably 10% by weight to 40% by weight, based in each case onthe weight of the impregnating composition, of comonomers, in particular(meth)acrylates having 7 to 15 carbon atoms in the alcohol radical.

According to a further expedient modification, the impregnatingcompositions according to the invention can preferably comprise 40% byweight to 80% by weight, preferably 50 to 70% by weight, of crosslinking(meth)acrylates, in particular ethylene glycol dimethacrylate,1,6-hexanediol dimethacrylate, triethylene glycol dimethacrylate,tetraethylene glycol dimethacrylate and/or trimethylolpropanetrimethacrylate; and 10% by weight to 50% by weight, preferably 20 to40% by weight, based in each case on the weight of the impregnatingcomposition, of comonomers, in particular (meth)acrylates having 7 to 15carbon atoms in the alcohol radical.

Of particular interest furthermore are impregnating compositions whichcomprise

30% by weight to 90% by weight, preferably 60 to 85% by weight, ofhydroxyalkyl (meth)acrylates, in particular 2-hydroxyethyl methacrylateand/or 2-hydroxypropyl methacrylate; and 1% by weight to 70% by weight,preferably 5% by weight to 50% by weight, based in each case on theweight of the impregnating composition, of crosslinking (meth)acrylates,in particular ethylene glycol dimethacrylate, 1,6-hexanedioldimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycoldimethacrylate and/or trimethylolpropane trimethacrylate.

For hardening, the impregnating composition preferably has at least oneinitiator. Suitable initiators are known to those skilled in the art.These include, inter alia, azo compounds, peroxy compounds, persulphatecompounds or azoamidines. Non-limiting examples are dibenzoyl peroxide,dicumene peroxide, cumene hydroperoxide, diisopropyl peroxydicarbonate,bis(4-tert-butylcyclohexyl)peroxydicarbonate,dipotassium persulphate,ammonium peroxydisulphate, 2,2′-azobis(2-methylpropionitrile) (AIBN),2,2′-azobix(isobutyramidine) hydrochloride, benzpinacol, dibenzylderivatives, methyl ethylene ketone peroxide,1,1-azobiscyclohexanecarbonitrile, methyl ethyl ketone peroxide,acetylacetone peroxide, dilauryl peroxide, didecanoyl peroxide,tert-butyl per-2-ethylhexanoate, ketone peroxide, methyl isobutyl ketoneperoxide, cyclohexanone peroxide, dibenzoyl peroxide, tert-butylperoxybenzoate, tert-butyl peroxyisopropylcarbonate,2,5-bis(2-ethyl-hexanoylperoxy) -2,5-dimethylhexane, tert-butylperoxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate,tert-butyl peroxyisobutyrate, tert-butyl peroxyacetate, dicumylperoxide, 1,1-bis(tert-butylperoxy)cyclohexane,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, cumylhydroperoxide, tert-butyl hydroperoxide, bis(4-tert-butylcyclohexyl)peroxydicarbonate and the free radical formers obtainable from DuPontunder the name ®Vazo, for example ®Vazo V50 and ®Vazo WS.

The initiators are usually used in an amount in the range from 0.01 to10% by weight, preferably from 0.05 to 5% by weight and veryparticularly preferably 0.1 to 1% by weight, based on the weight of theimpregnating composition.

In addition, initiator systems which are used for cold hardening arealso known. In general, these initiators have a redox system comprisingan accelerator and one or more of the peroxidic catalysts or initiatorsdescribed above. The amounts used here are frequently calculated suchthat the impregnating composition hardens within 5 to 120 minutes attemperatures in the range from 5 to 50° C.

The accelerator is usually used in an amount of 0.01 to 5% by weight,preferably 0.5 to 1.5% by weight, based on the weight of theimpregnating composition. The compounds particular suitable asaccelerators include, inter alia, amines and sulphimides, such as, forexample, tributylamine and benzoic acid sulphimide (saccharine).

Furthermore, organic metal salts which are usually used in the rangefrom 0.0001 to 0.5% by weight, based on the weight of the impregnatingcomposition, can serve as accelerators. These include, inter alia,copper octanoate.

Furthermore, the impregnating composition may comprise at least oneemulsifier. The total amount of emulsifier is preferably 0.1 to 15% byweight, in particular 2 to 10% by weight, based on the total weight ofthe impregnating composition. Particularly suitable emulsifiers arenon-ionic emulsifiers or mixtures thereof, in particular alkylpolyglycolethers, preferably having 8 to 20 carbon atoms in the alkyl radical and8 to 40 ethylene oxide units; alkylarylpolyglycol ethers, preferablyhaving 8 to 20 carbon atoms in the alkyl or alkylaryl radical and 8 to40 ethylene oxide units; ethylene oxide/propylene oxide copolymers,preferably block copolymers, for example having 8 to 40 ethylene oxideor propylene oxide units.

Of particular interest are especially impregnating compositions whosesolubility in the salt-containing cleaning composition is not more than6% by weight, particularly preferably not more than 4% by weight.Expediently, the solubility of hydroxypropyl methacrylate in thesalt-containing cleaning composition is preferably not more than 5% byweight, particularly preferably not more than 4% by weight. Thesolubility of the impregnating composition in the cleaning compositioncan be adjusted, inter alia, via the salt content of the cleaningcomposition.

Of particular interest are especially impregnating compositions whichpreferably have a density in the range from 0.9 g/l to 1.2 g/l,particularly preferably in the range from 0.95 g/l to 1.1 g/l.

According to an expedient modification of the process according to theinvention, the cleaning composition may have a density which differssubstantially from the density of the impregnating composition. Thedensity difference of cleaning composition and impregnating compositionis preferably at least 0.01 g/l, particularly preferably at least 0.05g/l and very particularly preferably at least 0.1 g/l.

By the measures according to the invention, the process canadvantageously be carried out continuously. In the context of theinvention, the expression “continuously” means that the cleaningcomposition can be separated continuously from excess impregnatingcomposition and reused.

Expediently, the cleaning composition can be circulated, the cleaningcomposition first being brought into contact with the impregnated metalsurface. As a result, excess impregnating composition is taken up by thecleaning composition and removed from the metal part.

The cleaning composition thus obtained can then be separated from theimpregnating composition. For this purpose, after being brought intocontact with the metal surface, the cleaning composition canadvantageously be passed into known apparatuses, such as, for example,oil separators, which separate off the impregnating composition from thesurface of the cleaning composition, or internals which collectimpregnating composition at the bottom of the cleaning composition. Thisseparation takes place particularly easily at a low solubility of theimpregnating composition in the cleaning composition.

The cleaning composition thus depleted with respect to the impregnatingcomposition can then be used again for recovering excess impregnatingcomposition.

The impregnating composition recovered by the recovery step can be useddirectly for impregnating further metal parts, it being possible, ifappropriate, to replenish monomers which remain in the cleaningcomposition owing to a solubility. In the case of preferred processes,however, only very small amounts are required for this purpose since thesolubility of these monomers can be kept low owing to the salt content.

The process according to the invention for recovering impregnatingcompositions can be used in particular in processes for impregnatingmetal parts. Accordingly, the present invention also relates toprocesses for impregnating metal parts.

Processes for impregnating metal parts have been carried out fordecades, these being described in detail, inter alia, in thepublications U.S. Pat. No. 6,712,910, U.S. Pat. No. 6,761,775, U.S. Pat.No. 5,518,632, DE 27 18 770, EP -A-0 014 062 and GB-1 547 801.

In general, the metal parts to be impregnated are first cleaned. Thiscan be effected, for example, by steam or by alkaline solutions, metalparts then being dried.

After the cleaning, the metal parts are preferably first exposed to adry vacuum. As a result of this, the air is removed from the pores. Themetal part is then impregnated with the impregnating composition. Forthis purpose, the metal part can be immersed in the impregnatingcomposition in vacuo. The vacuum may be, for example, 0.1 mbar to 100mbar, preferably 1 mbar to 20 mbar. The steps described above generallyrequire about 1 to 10 minutes, without it being intended to impose alimitation hereby.

Thereafter, the vacuum can be eliminated and the metal part immersed inthe impregnating composition can be stored at atmospheric pressure forin general about 2 to 20 minutes, preferably 5 to 10 minutes. As aresult of the atmospheric pressure, the pores are completely filled withimpregnating composition.

After the impregnation of the metal part, a major part of theimpregnating composition can be separated from the metal part bycentrifuging or similar methods.

The impregnated metal part is then brought into contact, according tothe invention, with a salt-containing cleaning composition. For thispurpose, for example, the cleaning solution can be sprayed onto themetal part. In addition, the metal part can be immersed in the cleaningcomposition. An additional cleaning effect can be achieved by passingthe cleaning composition under pressure over the surface of the metalpart.

Thereafter, excess cleaning composition can be removed from the metalpart by centrifuging or similar methods.

In a further step, remaining residues of the cleaning composition and ofthe impregnating composition can be eliminated by washing withwashwater. The washwater can likewise be separated from the metal partby centrifuging. Advantageously, the washwater can be used severaltimes, since any small residues of impregnating composition are removedfrom the metal surface by this step. In addition, owing to the lowcontent of impregnating composition, the washwater can be biologicallypurified.

After the excess impregnating composition has been removed from themetal part, the impregnating composition present in the metal part ishardened. Depending on the initiator system, this can be effected atroom temperature or by heating. Expediently, the metal part can beheated for example to a temperature in the range from 40 to 120° C.,preferably 60 to 90° C., in order to harden the impregnatingcomposition. Furthermore, the hardening can be effected at a temperaturein the range from 5° C. to 50° C.

Plants for carrying out the process described above correspondsubstantially to the plants which are described in the prior art citedabove, in particular in the publications U.S. Pat. No. 6,712,910, U.S.Pat. No. 6,761,775, U.S. Pat. No. 5,518,632, DE 27 18 770, EP-A-0 014062 and GB 1 547 801. These plants generally comprise a plurality ofcontainers in order to carry out the individual steps.

1. A process for recovering impregnating compositions, wherein animpregnated metal part is brought into contact with a salt-containingcleaning composition which comprises at least 0.7% by weight of at leastone salt.
 2. The process according to claim 1, wherein the salt is aninorganic salt.
 3. The process according to claim 1, wherein the salt isa phosphate.
 4. The process according to claim 1, wherein the cleaningcomposition is circulated.
 5. The process according to claim 1, whereinthe cleaning composition comprises 4 to 20% by weight of salt.
 6. Theprocess according to claim 1, wherein the impregnating compositioncomprises (meth)acrylates.
 7. The process according to claim 6, whereinthe impregnating composition comprises 5% by weight to 95% by weight ofhydroxyalkyl (meth)acrylates.
 8. The process according to claim 6,wherein the impregnating composition comprises 5% by weight to 75% byweight of crosslinking (meth)acrylates.
 9. The process according toclaim 6, wherein the impregnating composition comprises 5% by weight to60% by weight of comonomers.
 10. The process according to claim 1,wherein the impregnating composition comprises at least one initiator.11. The process according to claim 10, wherein the impregnatingcomposition comprises from 0.01 to 10% by weight of initiator.
 12. Theprocess according to claim 1, wherein the impregnating compositioncomprises at least one emulsifier.
 13. The process according to claim 1,wherein the solubility of the impregnating composition in thesalt-containing cleaning composition is not more than 6% by weight. 14.The process according to claim 7, wherein the solubility ofhydroxypropyl methacrylate in the salt-containing cleaning compositionis not more than 4% by weight.
 15. The process according to claim 1,wherein the density of the impregnating composition is in the range from0.9 g/l to 1.2 g/l.
 16. The process according to claim 1, wherein thedensity difference between the impregnating composition and cleaningcomposition is at least 0.05 g/l.
 17. A process for impregnating metalparts, comprising at least one step for recovering impregnatingcompositions according to the process of claim
 1. 18. The processaccording to claim 17, wherein the metal part is first impregnated withan impregnating composition, the impregnated metal part is then broughtinto contact with a salt-containing cleaning composition, the metal partis then cleaned with water and the impregnating composition present inthe metal part is hardened.
 19. The process according to claim 17,wherein the impregnating composition is brought into contact with themetal part at a pressure in the range from 0.1 mbar to 100 mbar.
 20. Theprocess according to claim 17, wherein the metal part is exposed to avacuum before the impregnation.
 21. The process according to claim 17,wherein excess impregnating composition is removed by centrifuging afterthe impregnation of the metal part.
 22. The process according to claim18, wherein the impregnating composition present in the metal part ishardened at a temperature in the range from 60 to 120° C.
 23. Theprocess according to claim 18, wherein the impregnating compositionpresent in the metal part is hardened at a temperature in the range from5 to 50° C.